Adhesive compositions are used extensively in the wood products industry to make composites such as chipboard, fiberboard, and related composite wood products. Adhesive compositions are also used to make engineered lumber composites. Traditionally, these composites have been made using a urea formaldehyde (UF) resin or a phenol formaldehyde (PF) resin. More recently, polymeric methylenediphenyl diisocyanate (PMDI) has been used to make these composites. UF resin, PF resin and PMDI are made from petroleum feedstock and can require high temperature conditions to facilitate cure. For example, heating the resin-wood mixture to temperatures exceeding 100° C., and often 200° C., while exerting pressure on the mixture in order to form the composite. These high-temperature conditions are problematic in certain structural (or engineered) lumber applications when UF and PF resins are used because it is often impractical to reach such high temperatures necessary to cure the adhesive due to the large size and inadequate heat transfer throughout the engineered wood composite. The high-temperature conditions are generally less problematic for PMDI resins because alternative heat transfer mechanisms can be used. However, PMDI resins are more costly that UF and PF resins. Thus, lower resin loadings must be used in the composite to make these composites on economical terms, but the lower loading of resin can itself be problematic for certain prior PMDI-based resins because it can be difficult to efficiently disperse small quantities of resin in the wood.
Recent environmental concerns emphasize the need for adhesive compositions that are environmentally friendly. Adhesive compositions frequently used in the wood products industry, however, are not environmentally friendly. For example, UF resin and PF resin contain the toxic and volatile chemical formaldehyde, which is harmful to the environment and can cause health problems for individuals. Furthermore, UF resin and PF resin require a petroleum feedstock. Thus, the need exists for adhesive compositions that reduce the need for petroleum feedstock, minimize use of toxic chemicals, and are amenable to the cure conditions and performance requirements for wood products.
In response to the need for environmentally friendly adhesive compositions, there has been renewed interest in using certain soy products to form adhesive compositions. However, there are multiple challenges in developing an adhesive composition from soy products. For example, the adhesive composition when cured to form a binder must have sufficient bond strength. The adhesive composition when cured to form a binder should, for certain applications, be sufficiently resistant to moisture. Another challenge is that the adhesive composition must have sufficient pot life so that it does not cure before being applied to components in the wood product. It is also important that the soy product be capable of production on large scale at economically feasible terms, and that it is amenable to cure conditions used to form the wood product.
Various reports describe efforts at developing an adhesive composition using certain soy products. U.S. Patent Application publication 2008/0021187 describes an adhesive composition containing urea-denatured soy flour. U.S. Pat. No. 7,416,598 describes an adhesive composition containing a protein ingredient and a modifying ingredient. Zhong and coworkers describe an adhesive composition containing certain soy protein material that has been modified. Zhong et al. in J. Appl. Polym. Sci. (2007) 103: 2261-2270. Yet, despite these efforts, the need exists for an environmentally friendly adhesive composition that meets the demands for widespread industrial application in the wood products industry.
The present invention addresses this need, and provides other related advantages.